In many uses of alkali hydroxides, excess of the alkali hydroxide is used, resulting in the formation of gels, sols, dispersions and solutions. This is especially the case for sodium hydroxide etchants of aluminum and alloys of aluminum. Aluminum is usually etched in a 15 wt. % to 30 wt. % solution of sodium hydroxide at temperatures above 50.degree. C. until about 20 or more grams of aluminum are etched per liter of etchant. The aluminum appears to be dissolved and seeding and cooling results in a very limited removal of sodium aluminate or aluminum hydroxide by filtration of the etchant. Since the etching rate decreases as the aluminum is etched into the sodium hydroxide solution, it is necessary to replace the etchant at relatively low concentrations of aluminum. The alloys of aluminum contain copper, silicon and other metals that are etched into the sodium hydroxide etchant and form a smut or slime or solution. To facilitate the etching process, additives of sulfur, amines and wetting agents are used in the sodium hydroxide etchant. It would be desirable that the etchant be continuously restored to maintain a desired milling rate and quality of the milled alloys. It is an objective of the instant invention to provide a continuous process for purification and restoration of alkali hydroxide etchants for aluminum and aluminum alloys.
It is possible to remove dissolved aluminum hydroxide from sodium hydroxide etchants by cooling and seeding the etchant. (See U.S. Pat. Nos. 4,136,026 and 4,372,805.) This process is, however, difficult to carry out, limited in the percentage of dissolved aluminum (sodium aluminate) that can be removed from the etchant, does not remove soluble impurities, requires hours for effecting separation and requires a relatively large investment in crystallization, filtration and evaporation equipment.
It is, also, known that the alkali hydroxide etchants can be neutralized or acidified with an acid, such as, sulfuric acid, and the aluminum hydroxide separated by filtration. The resulting salt solution can be disposed of as waste or electrodialytically converted to an acid and alkali hydroxide as disclosed in my U.S. Pat. No. 4,636,288. This two-step process requires that the salt solution filtrate be acidic in electrodialysis which increases the electrical cost for removing alkali cations and requires the use of a three compartment cell to prevent fouling of the cation membranes with magnesium, calcium and other multivalent metal cations in the etchant and salt solution. It would be preferable that the alkali hydroxide etchant be restored in a continuous one-step electrodialytic process where the unwanted aluminum hydroxide and other multivalent metal hydroxides are insolubilized in the pH range of 13 to 7 and removed as solids from an electrolyte and the alkali hydroxide be returned to the etcher as a solution of substantially pure alkali hydroxide. It is an object of this invention to provide a one-step continuous process for reforming alkali hydroxide etchants of aluminum and alloys of aluminum.
Electrodialysis is a well-known art (see U.S. Pat. Nos. 4,325,792; 4,439,293 and 4,626,288, the disclosures of which are incorporated by reference). Electrodialysis is the transport of ions through ion permeable membranes as a result of an electrical driving force. The process is commonly carried out in an electrochemical cell having a catholyte compartment containing a cathode and a catholyte and an anolyte compartment containing an anode and an anolyte, the catholyte and anolyte compartments being separated by ion permeable membranes. The electrotransport of sodium and other alkali metal cations through cation permeable membranes is a known art. However, prior art does not provide a means for insolubilizing in an electrolyte of an electrodialytic process aluminum and other metal hydroxides and metallic oxides acidic in an alkali hydroxide solution, gel, sol or colloidal dispersion feed to the electrodialytic process.
Many of the hydroxides of heavy metals, such as aluminum, lead, tin, zinc, gallium and tungsten are soluble or appear to be soluble in excess of sodium or potassium hydroxide. This has been attributed to the formation of salts, the hydroxides behaving as amphoteric substances and giving either OH.sup.- or H.sup.+ ions according to the condition of the experiment. For example, when aluminum hydroxide is dissolved in sodium hydroxide, sodium aluminate is supposed to be formed. It is possible, however, that the solution of the aluminum is not so much a matter of compound formation as of peptization of the hydroxide to form a sol, gel or colloidal dispersion. It is, of course, difficult to mechanically separate the multivalent metal hydroxide from an alkali hydroxide gel, sol, solution or colloidal dispersion.
When a metal has several oxides, the basic properties of the hydroxides become less pronounced as the valency of the metal increases. When a certain limit of valency is reached, the basic properties disappear almost completely and salt formation does not take place to an appreciable extent. The acidic oxides are formed only by those metals which can exert a high valency and thus combine with several oxygen atoms. The acidic tendency is almost invariably in the quinquevalent and higher valence of the metal. Hence metals, in the right-hand half of the periodic table give acidic oxides that form salts with alkali metal cations. These metallic oxides acidic, such as molybdic, tungstic, uranic, vanadic, niobic and tantalic, cannot always be isolated in pure form by neutralization of the salts as they are frequently converted to anhydrides or polymerized or dissolved in excess of the neutralizing acid. These acids are made from a lower oxide of the metal by heating the oxide with alkali, usually in the presence of an oxidizing agent. The excess alkali is removed by neutralization with an acid to form a soluble alkali salt.